The present application relates to electromagnetic free point tools. More particularly, methods and devices are provided for determining the free point of a downhole pipe in a wellbore by magnetic permeability measurements utilizing amplitude or phase change and lock-in amplifiers.
During the course of drilling oil and gas wells, the drill pipe can become stuck for any number of reasons such as differential well bore pressure, key seating, sand bridging, well bore collapse, and swelling of the borehole to name a few. The stuck drill pipe must then be removed before the well bore can be repaired and drilling operations continue. The first step in removing the stuck pipe is to determine where in the well it is stuck so that the section of pipe above this point (the free pipe) can be removed.
There are a number of methods that have been used to determine the location of the free point of the pipe (or alternatively, the stuck point of the pipe). A low resolution method involves measuring how much the pipe twists or stretches when a particular torque or force is applied to the pipe at or near the surface. Knowing the elastic properties of the drill pipe allows one to calculate the length of pipe that is free to within several hundreds of feet. To pinpoint the free point (or stuck point) within this interval, a “free point tool” is employed. The most common type are mechanical tools, which are lowered within the pipe with temporary anchors that extend to lock both ends of the tool to the borehole. Sensors within the tool detect relative movement of the tool ends when the drill pipe is twisted or stretched. If the tool is anchored below the stuck point no differential motion is detected. Gradually, the location of the stuck point is located once the tool detects differential movement. The same procedure could also be repeated in reverse, moving the tool downwards as well.
Another common type of tool for determining differential movement is similar to the aforementioned tool except that strong springs are used to hold the tool in position.
These methods suffer from several disadvantages. First, it is rather time consuming to move the tool, set the anchors, release tension on the cable, stress the drill pipe, and measure the relative motion (if any). This time-consuming procedure must be repeated until the stuck point is located. Second, it is not always straightforward to interpret the results of a given measurement so that highly experienced personnel are often required to interpret the measurements.
Previous attempts have been made to use an electromagnetic free point tool. For example, U.S. Pat. No. 4,708,204 describes an example of such a wireline tool. Such tools basically consist of two coils separated by a fixed distance on a mandrel. An oscillating voltage would be applied to the first coil (i.e. a transmitter coil) and the induced current in a second coil (i.e. a receiver coil) would be measured relative to the applied voltage of the first coil. By making continuous measurements over an interval of interest, a log of receiver coil phase or amplitude relative to the transmitter coil phase may be generated. A second log may be generated during a second pass of the tool after the pipe has been stressed. The two logs can then be compared so as to determine the stuck point of the pipe, as the two logs will be more or less identical below the stuck point and will diverge above the stuck point.
Although such tools functioned reasonably well when used in bare drill pipe, these tools failed if the drill pipe was located inside of a second ferromagnetic casing pipe. In particular, such tools failed because the operating parameters for the tool were optimized for a single string of pipe. The additional pipes perturbed the response sufficiently that the tool response to the desired pipe parameters would be obscured.
Accordingly, there is a need for improved free point tools that address one or more disadvantages of the prior art, and specifically, a free-point tool robust enough to accurately identify the stuck point of a pipe string in both cased and uncased holes is desired.